On Saturday 03 July 2010 2:11:37 pm David Menendez wrote:
> > {-# LANGUAGE MultiParamTypeClasses #-}
> > {-# LANGUAGE FunctionalDependencies #-}
> > {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
> >
> > class C a b c | a -> b, a -> c where
> > op :: a -> b -> c
> >
> > instance C Bool a a where op _ = id
> >
> > main = putStrLn $ op True "done"
> >
> >
> > In this case, I've (arbitrarily) chosen the Bool instance to be a no-op
> > and pass through the types. Because the dependent types are part of the
> > declaration header I can use type variables for them.
>> That's really weird. In particular, I can add this line to your code
> without problems:
>> foo = putStrLn $ if op True True then "done" else "."
>> but GHC complains about this one:
>> bar = putStrLn $ if op True True then op True "done" else "."
>> fd.hs:14:0:
> Couldn't match expected type `Bool' against inferred type `[Char]'
> When using functional dependencies to combine
> C Bool [Char] String, arising from a use of `op' at fd.hs:14:38-51
> C Bool Bool Bool, arising from a use of `op' at fd.hs:14:20-31
> When generalising the type(s) for `bar'
>> On the other hand, this is fine, but only with a type signature:
>> baz :: a -> a
> baz = op True
>> I don't think this is an intended feature of functional dependencies.
Indeed. That instance declaration doesn't really make sense, and should
probably be rejected. The functional dependencies on C say that b and c are
dependent on a, so for any particular a, there should be exactly one b and one
c such that C a b c is an instance.
Then the instance declares infinitely many instances C Bool a a. This is a
violation of the fundep. Based on your error message, it looks like it ends up
treating the instance as the first concrete 'a' it comes across, but who
knows?
-- Dan